This identification is often achieved by multispectral imaging; the latter is based on a spectral decomposition of each point of the observed scene.
Accordingly, there exist several techniques. The most widespread implement a spectro-imager or colored filters. Other much less commonplace techniques use a Fourrier transform spectrometer to obtain the spectral decomposition. Multispectral and hyperspectral imagers operate mainly in passive mode: the light backscattered by the scene originates from solar or infrared radiation, from the inherent emission of the objects of the scene. Multispectral imaging covers a wide field of applications. It is used in geology for the identification of certain minerals and in particular for the detection of precious metals. It is also of interest in the oil industry for the detection of hydrocarbon and gas traces by direct observation or by observation of their effects on vegetation. In the field of agriculture, multispectral imaging makes it possible to track the development of plantations, to detect the presence of parasites or to evaluate the level of irrigation. It should be noted that multispectral imaging is penetrating ever more into ecological fields for the effective tracking of deforestation or reforestation of the terrestrial surface for example. These imagers are often carried aboard observation airplanes or satellites. The Landsat satellites, for example, carry 7 radiometers on board. It may be noted finally that multispectral imaging is beginning to find applications in the field of defense and security for zone monitoring notably. Its main objective is to circumvent the camouflage measures taken by the military in the visible and thermal regions of the optical spectrum. In this approach, one in fact gambles that the spectral analysis band is sufficiently large and resolved for each object to have a unique signature.
Although less widespread, it is known to resort to monochromatic polarimetric imaging for applications in microscopy, in the characterization of surfaces or interfaces, in the detection of stresses and in biology for tracking the evolution of cancerous cells for example. This type of imaging makes it possible to analyze the state of polarization of the light backscattered by an object. Polarimetric imagers often operate in active mode. A system which comprises a laser source illuminating the scene of interest and a detection system sensing the laser flux back-scattered by the scene and making it possible to form a two-dimensional image is called an active imager. It is necessary to fix or to control the state of polarization of the illumination light.
There exist diverse acquisition procedures. The most exhaustive is Muller imaging in which the state of polarization of the backscattered light is analyzed in a rigorous manner as a function of the state of polarization of the incident light. It requires the acquisition of 16 images, each associated with an element of the Muller matrix (4×4). Muller imaging is useful when the object to be characterized produces on the light, in addition to a pure depolarization, a change of polarization state due to a geometric, birefringence or optical activity effect. It finds applications in the biomedical field and in microscopy. More pragmatically, it is often sufficient to produce a polarimetric contrast image. In this case the state of polarization of the incident light is fixed (often a linear polarization) and the light backscattered by the object in accordance with a parallel polarization is analyzed; and then the light backscattered by the object in accordance with an orthogonal polarization is analyzed. Thus, the number of images to be acquired is limited to two without however losing an enormous amount of information, provided that the object analyzed is purely depolarizing. This is often the case when scenes on a human scale are considered.
In the foregoing, whether it is by multi-spectral imaging or by polarimetric imaging, the images are acquired sequentially over time.
These techniques are no longer efficacious if the imaged scene varies over time as for example in the cases of a scene which comprises objects in motion, or in the presence of atmospheric disturbances or else when the illumination of the scene is pulsed with energy fluctuations from one pulse to another.